Planet

REFERENCE MATERIAL ABOUT PLANETS

The original Greek word meant, simply, "wanderer," referring to points of light that moved across the nighttime background of fixed stars. The word has evolved since then, since, the original Greek definition taken literally would include more than 100,000 bodies orbiting the Sun.

In 2006, the International Astronomical Union, in demoting Pluto from a planet to a dwarf planet, adopted this definition: A "planet" is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighborhood around its orbit.

Pluto does not satisfy condition (c) of the definition.

Astronomers have also discovered more than 200 planets orbiting other stars.

The maximum mass of a planet is generally considered to be about 13 times that of Jupiter. Objects larger than that reach temperatures hot enough for some fusion reactions to begin and are called brown dwarfs.

However, some astronomers distinguish stars from planets not by mass by how they formed. Planetary systems form in a two-step process. First, a cloud of gas collapses under gravity into one, two and sometimes three stars. Then, a disk of leftover gas and dust coalesces into planets.

Some astronomers now think that the initial star-forming gas collapse can result in planet-size objects and it may be possible for the leftover dust and gas can coalesce into a brown dwarf-size planet. Since astronomers cannot go back in time to see exactly how something formed, Gibor Basri, a professor of astronomy at the University of California, Berkeley, invented the word, "planemo," a shortening of "planetary mass object" (and pronounced PLAN-uh-moh), to describe any object with a mass less than 13 Jupiter masses regardless of how it formed.